This protocol will test the efficacy of exogenous melatonin on initiating and sustaining a full-duration daytime sleep episode and subsequently assess the effects of this sleep on nighttime performance. Melatonin (0.5 mg and 15 mg) and placebo will be tested in subjects attempting an 8-hr daytime sleep episode following a full night sleep. The effect of melatonin-induced daytime sleep on nightime performance will be assessed in 36-hours of sleep deprivation.